外观设置改为半屏布局；背景图仅保存后生效，并支持媒体库选用。

Co-authored-by: Cursor <cursoragent@cursor.com>

frontend/lib/api.ts

@@ -2268,10 +2268,11 @@ export async function apiBrandFromMedia(slot: BrandSlot, attachmentId: number):
 }
 
 /** 上传站点背景并立即绑定前台或后台 */
+/** 超管上传站点背景图：只落盘返回 URL，需再 PUT settings 才绑定并广播 */
 export async function apiUploadSiteBackground(
   surface: "site" | "admin",
   file: File
-): Promise<PublicSettings> {
+): Promise<string> {
   const form = new FormData();
   form.append("surface", surface);
   form.append("file", file, file.name || "background.jpg");
@@ -2282,7 +2283,26 @@ export async function apiUploadSiteBackground(
   });
   const data = await res.json().catch(() => ({}));
   if (!res.ok) throw new Error(data.error || "上传背景图失败");
-  return normalizePublicSettings(data as Partial<PublicSettings>);
+  const url = String(data.url || "");
+  if (!url) throw new Error("上传背景图失败");
+  return url;
+}
+
+/** 把本人媒体库图片复制进 backgrounds，不写入站点设置 */
+export async function apiBackgroundFromMedia(
+  surface: "site" | "admin",
+  attachmentId: number
+): Promise<string> {
+  const res = await fetchWithRefresh("/api/admin/upload/background/from-media", {
+    method: "POST",
+    headers: clientHeaders({ "Content-Type": "application/json" }),
+    body: JSON.stringify({ attachment_id: attachmentId, surface }),
+  });
+  const data = await res.json().catch(() => ({}));
+  if (!res.ok) throw new Error(data.error || "选用媒体库图片失败");
+  const url = String(data.url || "");
+  if (!url) throw new Error("选用媒体库图片失败");
+  return url;
 }
 
 // ===== 板块（客户端，供角色授权弹窗勾选） =====

frontend/lib/bg.ts

@@ -50,6 +50,11 @@ export function normalizeBgMode(raw: string | null | undefined): BgMode {
   return MODE_SET.has(m) ? m : DEFAULT_BG_MODE;
 }
 
+/** 媒体库条目是否可作为背景图（JPEG / PNG / WebP） */
+export function mediaURLFitsBackground(url: string): boolean {
+  return /\.(jpe?g|png|webp)(\?|$)/i.test(url);
+}
+
 export function bgConfigFromSettings(s: SiteBgInput | null | undefined): SiteBgConfig {
   return {
     bg_site_url: normalizeBgURL(s?.bg_site_url),